In the past recent years, various types of machines for felling trees have been developed. A typical example is the tree-felling tractor used in large scale tree-harvesting operations, which is a self-propelled all terrain vehicle specifically designed to operate in forest areas under harsh conditions. The vehicle is provided with an articulated boom carrying at its end a cutting mechanism, designated in the art as "cutting head". The articulated boom is hydraulically operated and it can be extended and retracted, raised, lowered or swung through 360 degrees allowing to bring the cutting head against a standing tree to be cut. The cutting head is provided with a cutting tool for cutting the tree, as well with a grappling mechanism to grasp the tree once the cutting phase is completed. The felled tree can then be manipulated to be deposited on the ground or on a trailer for ultimate transportation to a mill.
As it is known to those skilled in the art, efficient operation of the cutting head is essential to obtain good harvesting rates. For that reason considerable efforts have been devoted by the past to the design of a highly efficient cutting head that is reliable, versatile and inexpensive to build and to operate. Among the various designs that have been proposed, the cutting head disclosed in the U.S. Pat. No. 4,446,897 granted on May 8, 1984 to Koehring Canada Limited, has found wide acceptance in the industry. The approach in this patent is to provide a cutting head with a circular saw mounted to a supporting structure that also carries a pair of hydraulically operated arms. In use, the cutting head is placed against a standing tree by swinging sideways the boom of the tree-felling vehicle, and when the cut of the tree is nearly completed, the arms are closed to grasp the tree, which is then carried away.
This prior design has not been completely satisfactory in providing optimum tree-cutting performance at minimum cost. Its main disadvantage is that it is rather heavy and it can be used only with large tree-felling vehicles. Furthermore, it is complex to operate requiring skill and experience from the operator of the machine to close the power arms only when the cutting phase is almost completed. If the arms are closed too early, the tree may splinter because of the high bending efforts exerted by the arms. On the other hand, closing the arms too late results in an inability to retain the tree in the cutting head.
In another known design, the cutting head is equipped with a hydraulically or pneumatically operated shearing device having a pair of knives that shear-off the tree instead of cutting it with a rotary tool. These cutting heads are inexpensive and operate relativeley faster, however, they are known to cause wood splintering especially if the knives do not meet exacly in the same plane, which may be the result of wear or of improper adjustment. The wood splinter damages are to be avoided because the splinted wood portion of the tree must be cut-off and treated as waste.
Generally speaking, the prior designs have not been able to meet all the requirements to ensure an easy and trouble-free operation during adverse tree-harvesting conditions in forest areas.